Improve compile-time error handling.

pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.dill_library_builder;
 
 import 'package:kernel/ast.dart' show
     Class,
     ExpressionStatement,
     Field,
@@ skipping 40 matching lines @@
 
   DillLibraryBuilder(Uri uri, this.loader)
       : uri = uri, super(uri);
 
   get scope => internalError("Scope not supported");
 
   Uri get fileUri => uri;
 
   void addClass(Class cls) {
     DillClassBuilder classBulder = new DillClassBuilder(cls, this);
-    addBuilder(cls.name, classBulder);
+    addBuilder(cls.name, classBulder, cls.fileOffset);
     cls.procedures.forEach(classBulder.addMember);
     cls.constructors.forEach(classBulder.addMember);
     for (Field field in cls.fields) {
       if (field.name.name == "_redirecting#") {
         // This is a hack / work around for storing redirecting constructors in
         // dill files. See `buildFactoryConstructor` in
         // [package:kernel/analyzer/ast_from_analyzer.dart]
         // (../../../../kernel/lib/analyzer/ast_from_analyzer.dart).
         ListLiteral initializer = field.initializer;
         for (StaticGet get in initializer.expressions) {
@@ skipping 12 matching lines @@
     }
   }
 
   void addMember(Member member) {
     String name = member.name.name;
     if (name == "_exports#") {
       // This is a hack / work around for storing exports in dill files. See
       // [compile_platform.dart](../compile_platform.dart).
       print("$uri: _exports# not implemented yet.");
     } else {
-      addBuilder(name, new DillMemberBuilder(member, this));
+      addBuilder(name, new DillMemberBuilder(member, this), member.fileOffset);
     }
   }
 
-  Builder addBuilder(String name, Builder builder) {
+  Builder addBuilder(String name, Builder builder, int charOffset) {
     if (name == null || name.isEmpty) return null;
     members[name] = builder;
     if (!name.startsWith("_")) {
       exports[name] = builder;
     }
     return builder;
   }
 
   bool addToExportScope(String name, Builder member) {
     return internalError("Not implemented yet.");
   }
 
   void addToScope(String name, Builder member) {
     internalError("Not implemented yet.");
   }
 
   KernelInvalidTypeBuilder buildAmbiguousBuilder(
       String name, Builder builder, Builder other, int charOffset) {
     return new KernelInvalidTypeBuilder(name, charOffset, fileUri);
   }
 }